Tracer controlled machine tool



June 20, 1950 T. A. WETZEL 2,511,956

TRACER CONTROLLED MACHINE TOOL Filed Feb. 28, 1944 5 SheetsHSheet 1 7/ m We D o 0 c w i 5 52' M/l/EA/TOQ Thzobam A. hfrrzzz ATTO/QNLV June 20, 1950 1 A. WETZEL 2,511,956

TRACER CONTROLLED IIACHINE TOOL Filed Feb. 28, 1944 5- Shoets-Sheet 2 l/Vl/E/V TOR THEODORE A. M'TZzL June 20, 1950 1'. A. WETZEL 2,511,956

TRACER CONTROLLED MACHINE TOOL Filed Feb. 28, 1944 5 Sheets-Sheet 3 o 222 9 9- 4 3/3 324 373 320 2/0 o o INVENTOQ EJ000954 #512511. 230 233 :235 5v a ATTORNEY Patented June 20, 1950 TRACER CONTROLLED MACHINE TOOL Theodore A. Wetael, Milwaukee, Wis., assignor to Kearney & Tracker, West Allis, Wis., a corpora- 1 'tion of Wisconsin Another object is to provide an improved actu-' ating and controlling mechanism for a tracer controlled machine tool in which control of machine movements is eifected by reversing the polarity of the electric current that provides power for causing the machine movements.

Another object is to provide machine tool actuating and controlling mechanism in which a driving motor is provided with energy from a motor generator set, the output of which is controlled in accordance with the cutting action required.

Another object is to provide a tracer controlled machine tool in which a motor generator set provides actuating current for operating the ins-' chine, the output of the generator being controlled by electronic tubes arranged to respond to tracer mechanism.

Another object is to provide a tracer controlled machine tool'in which a tracer system is arranged to act upon electronic tubes that control the excitation of a generator which provides electric current for eilecting movements in accordance with the requirements of the tracer in following a guiding pattern.

Another object is to provide a tracer controlled machine in which actuating'electric current for the machine is derived from a generator that is excited by an intermittent current, the frequency of interruption of the current being regulated by a tracer mechanism in accordance with the requirements of a guiding pattern,

Another object of the invention is to provide a tracer controlled machine tool in which the rate of change of movement of the machine elements functions to modify the controlling action of the tracer.

Another object is to provide in a tracer controlled machine tool, control means responsive to changes in the rate of movement of machine elements and arranged to react upon the controlling tracer mechanism selectively to accentuate or diminish the effect of the tracer in causing Application February 2c, 1944, Serisl'No. 524,102

0 25 claim. (01. 318-182) 2 machine tool in which control actions eilecting deceleration of relative movement of machine elements are of greater magnitude than control actions eifecting acceleration thereof, whereby a damping eifect is provided to prevent hunting.

Another object is to provide a tracer controlled machine tool in which a brake is applied selectively to assist decelerating actions whereby overtravel of machine elements and hunting is prevented.

Another object is to provide an improved tracer controlled attachment for a standard milling machine having means for coordinating the movements of the milling machine table with those of the tracer controlled cutter spindle.

Another object is to provide a tracer controlled machine tool in which a tracer is arranged to effect movement of one element of an electronic tube, the tube being connected to control power means eifecting relative movement of machine elements.

Another object is to provide a tracer controlled trol system in which a controlled action occurs a in accordance with the amount and direction of changes in movement of the machine elements. ll

Another object is to provide a tracer controlled movement of a control member and a modifying effect depending upon the rate of the controlled I action operates to change the influence of the control member in effecting the controlled action.

According to this invention a tracer controlled machine tool is arranged to be actuated by electric current of reversible polarity furnished from a generator, the output of which is regulated under the control of tracer mechanism. The generator may be driven by the spindle driving motor in the manner of a motor-generator set, a separate feeding motor being provided to effect the controlled feeding action through operation by current from the generator. The generator excitation field is under the control of tracer mechanism acting through electronic tubes and amplifiers in manner to provide current of either polarity for operating the feeding motor in either direction as required. The apparatus may be in the form of an attachment for a standard milling machine, in which event the feeding motor moves the cutter spindle and another motor is arranged to effect line feeding and cross feeding .movement of the table, the two feeding motors various angles to the work. According to one arrangement, the tracer actuates a movable condenser plate to unbalance a pair of electronic tubes that are connected through the amplifier to the control field of the generator, the arrangement being such that when the condenser element is moved in the one or the other direction, the generator is caused to generate electric current of one or the other polarity characteristic and of strength proportional to the deflection of the tracer. Accordingly, the tracer feeding motor is' operated in the one or the other direction in accordance with the requirements of the tracer movements to effect the desired feeding action of the cutter spindle. Theaction of the feeding motor is modified-by an electrical feedback arrangement which is responsive to changes in the rate of action upon the feeding motor and functions to modify the efiect of the tracer control system. The feedback system may be adjusted selectively to function according to various modes of operation whereby the feedback efiect results in accentuating or in diminishing the effect of the tracer system to a predetermined extent. To

obviate oscillationsin the feed drive, the decelerating action upon the feeding motor may be greater than the accelerating action, thereby preventing overrunning and providing a damping effect. As a further means for preventing overrunning, a brake associated with the spindle feeding motor is controlled in manner to be engaged during decelerating actions of the motor, both .to prevent overrunning and to damp out oscillations. Instead of the spindle feeding motor being operated directlyfrom the generator,the controlling effect may be accomplished by means of magnetic clutches actuated by the generator current. As another variation, the tracer may be arranged to regulate the frequency of vibration of contact points through which current flows to the generator excitation fields. A further modified type of control utilizes an electronic tube having a movable member actuated by the tracer. In place of the motor-generator set there may be substituted saturable core transformers that are controlled by the electronic tubes and that operate through rectifier elements to provide unidirectional currents of selected polarity to the spindle feeding motor.

The foregoing and other objects of this invention, which will become more fully apparent from the following description, may be achieved by means of the exemplifying apparatus depicted in and described in connection with the accompanying drawing in which:

Figure 1 is a view in perspective of a milling machine of the standard knee and column type equipped with a tracer controlled milling attachment embodying the present invention;

Fig. 2 is a schematic circuit diagram showing the electrical actuating and controlling system of the tracer controlled apparatus shown in Fig. 1;

Fig. 3 is another schematic circuit diagram showing a modified form of the control circuit and a different arrangement of the parts;

Fig. 4 is another schematic diagram showing part of the control circuit of Fig. 3 as applied to a machine having a magnetic clutch driving arrangement;

Fig. 5 is another schematic circuit diagram showing a ,modified form of the invention in which the tracer is arranged to control the frequency of interruption of a control circuit for regulating the action of the feeding mechanism;

Fig. 6 is another schematic circuit diagram showing a modification of the invention in which the tracer is arranged to actuate a movable element of an electronic tube, the power for feed ing movements being derived from saturable core transformers controlled from the electronic tube;

Fig. 7 is a view partly in end elevation and partly in transverse section, of a direction and acceleration responsive switch that is associated with the feed driving mechanism, parts of the switch having been broken away to better show the interior;

Fig. 8 is a view in longitudinal section through the switch mechanism, taken along the line 8-! in Fig. 7;

Fig. 9 is a schematic circuit diagram of part of the tracer controlled. actuating circuit, showing an application of the switch shown in Figs. 7 and 8;

Fig. 10 is a schematic circuit diagramgenerally similar to Fig. 9 but showing another application of the direction and acceleration responsive switch; and

Fig. 11 is still another similar schematic circuit diagram showing a further application of the direction and acceleration responsive switch.

The particular tracer controlled machine tool shown in the accompanying drawing as exemplifying a preferred embodiment of the present invention, is in the form of an attachment mounted on a milling machine of standard construction, although it is to be understood that the invention may be embodied with equal advantage in a unitary machine structure especially designed for performing tracing operations.

Referring more specifically to the drawing, and particularly to Fig. 1 thereof, the illustrative apparatus there shown is embodied in an attachment 20 that is supported upon the upper forward part of a column 2! of a standard knee type milling machine by means of the usual overarms constituted by parallel round bars 22 that project from the front of the column in well known manner. The column 2| of the machine also supports the usual work holding elements including a vertically adjustable knee 23 thatcarries a saddle 24 slidably mounted on its upper surface for transverse movement toward or from the column. The saddle 24 in turn carries a work table 25 that is slidably mounted thereon for longitudinal feeding movement in the usual manner.

The tracing attachment 20 comprises a supporting frame 21 that presents a depending cutter spindle 28 and a parallelly disposed tracer sleeve 29, both arranged for vertical sliding movement in the frame. The spindle 28 and sleeve 29 are spaced sumciently, in this casein direction of the longitudinal movement of the work table 25, to support a cutter 30 and a tracing point 3|, respectively, for cooperation with a work piece 32 and a pattern 33 that may be clamped or otherwise secured in tandem relationship on the table.

The cutter spindle 28 is 'rotatably driven by means of a motor 35 mounted vertically at the top of the attachment frame 21 and operatively connected to the spindle by means of a V-belt 36 cooperating with a pair of complementary stepped pulleys 31 and 38, associated with the motor and the spindle, respectively. The stepped pulleys constitute a speed adjusting transmission mechanism, the motor being slidably mounted in the attachment frame 21 to provide for tightening the belt. When the machine is in operation, the cutter spindle 28 and its cutter 30 are adjusted in vertical direction automatically in response to the controlling action of the tracer point 3| in traversing the pattern 33, the tracer sleeve 29 being moved simultaneously with the spindle 28 in the manner of a follower mechanism.

In executing a copying operation, the table 25 is fed longitudinally back and forth in a line feeding movement, the saddle being fed transversely at each reversal of table movement to provide a cross feeding movement, the combination resulting in a series of closely spaced longitudinal cutting strokes that are controlled vertically by the tracer in accordance with the contour of the pattern being followed. Although the usual power feeding mechanism of the milling machine may be utilized for eifecting the line feeding movement, it is preferable that the table be operated by a separate motor 44 that may be mounted on the end of the table as shown in Fig. l. The table driving motor" is adapted to be interconnected electrically with the tracer controlled mechanism in manner to control the table rate. As shown diagrammatically in Fig. 2, the motor 40 is connected to a table driving screw 42 of the usual type by suitable gearing 43 in manner to rotate it in either direction selectively, the machines usual power feed controlling clutch being preferably locked in disengaged position to avoid jamming of the mechanism through accidental engagement of both drives at once.

Control of the table feeding motor to cause it to effect successive line feeding movements in opposite directions automatically is provided by means of a reversing limit switch 44 conveniently mounted on the front edge of the table 25 and arranged to be operated by a trip rod slidably mounted for movement longitudinally of the table along its forward edge. The trip rod 45 is fitted with adjustably mounted stop dogs 48 and 41 disposed to engage an intermediate fixed abutment 48 carried by the saddle 24, the arrangement being such that when either of the dogs engages the abutment, the rod 45 is caused to move relative to the table 25 thereby actuating the switch 44 and causing the table motor 40 to reverse. The rod 45 is slidably mounted at its outer end in a lug 48 projecting from the table and the reversing dogs 46 and 41 are ordinarily positioned on the rod in manner to effect reversal of line movement when either extreme edge of the pattern 33 passes under the tracer point 3|.

For effecting cross feeding of the saddle 24 at each point of reversal of'the table 25, there is I provided an electromagnetic Jogger ratchet mechanism 50 that is mounted on the front of the knee 23. This mechanism comprises a ratchet wheel 5| that is connected to turn a screw shaft 52 constituting the usual saddle traversing screw. A cooperating pawl 53 is pivotally carried at the upper end of a. vertically disposed slidably mounted armature plunger 54 in position to engage with the ratchet wheelil. The armature plunger 54 is arranged to cooperate with an actuating solenoid 55 in such manner that when the solenoid is energized, the armature is drawn downward and the pawl 53 operates to turn the ratchet wheel 5| and the screw 52 a suiiicient distance to effect the desired cross-feeding movement. Re-

turn movement of the pawl 53 to its upper position after the solenoid has been de-energized. is effected by means of a coil spring 56 disposed to encircle the armature plunger 54 and arranged to exert'force upwardly against an enlarged upper end of the plunger which also serves to pivotally carry the pawl 53.

The cross feed solenoid 55 receives energizing current from the reversing switch 44 through a flexible cable 51, the switch 44 being connected with the table driving motor 40 by a similar cable 53. Operation of the motor 40 is controlled through a cable 59 that connects with a control cabinet 60 that may be-placed conveniently on the floor at the side of the machine, as shown in Fig. 1. The necessary connections from the control cabinet to the electrical apparatus of the attachment 20 is similarly effected through an appropriate flexible cable 6|.

In order to provide for positioning the cutter 30 and the tracer point 3! angularly for tracing a pattern having an undercut portion at one side, the attachment 20 is mounted upon the supporting overarms 22 by means of a clamping arrangement which permits pivotal movement thereof about a horizontal axis parallel with the longitudinal axis of the table 25. This structure includes a hollow cylindrical clamping member 64 secured to the overarms and fitted about a complementary cylindrical portion of the supporting frame 21 constituting the body of the attachment and carrying the spindle and tracer together with their associated mechanism. A scale 66 on one end of the clamping member 64 may be read against a cooperating zero mark 61 on the frame 21 in adjusting the spindle 2B and tracer sleeve 29 to the desired angular position. After the adjustment has been completed, the clamping cylinder 64 may be tightened upon the frame 21 in well known manner to retain it against further movement.

The mechanism for effecting the automatic vertical feeding movement of the cutter spindle 28 and the tracer sleeve 28 includes a highly responsive feed-motor 10 that is mounted in the lower part of the attachment frame 21 as shown diagrammatically in Fig. 2 and is controlled electrically by movement of the tracer point 3|? As indicated, the motor 10 is provided with a relatively long shaft H carrying pinions I2 and 13 fixed on each of its ends, respectively. The pinion 12 is disposed to mesh with a rack 14 formed on a vertically slidable quill 15 within which the cutter spindle 28 is rotatably mounted. The other pinion l3 meshes with rack teeth 16 formed on the vertically slidable tracer sleeve 29, the arrangement being such that the cutter spindle and the tracer sleeve are moved simultaneously in the same direction by'any rotary movement oi the motor shaft 1 l The motor 10 is operated in the one or the other direction by reversing the polarity of its current supply, the polarity and strength of its energizing current being controlled in response to deflections of the tracer point 3| in accordance with the contour of the pattern. Several forms of the control mechanism for providing reversible polarity energizing current to the actuating mechanism are illustrated in the accompanying drawings and described in detail hereinafter.

motor is provided from a generator "that operates under the control of th tracer mechanism. The generator '80 may conveniently be associated with and driven by the spindle driving motor 35, it being in this instance mounted directly abov the motor on the same shaft to constitute a motor-generator set. Since the motor 35 operates at substantially constant speed, the

generator 80 is driven uniformly and the polarity and strength of its output current may be regulated by controlling the generator field in re-- sponse to the operation of the tracer mechanism.

In the particular apparatus here illustrated, a tracer box 8I carries a tracer mechanism 82 of the variable capacitance type that is provided with a movabl condenser plate 83 arranged to I be actuated by the tracer point 3| in operating over the pattern. Two stationary plates 84 and 85 are disposed at the opposite sides of the movable plate 83 and are electrically connected respectively with electronic tubes 85. and 81 mounted in the control cabinet '80. Movementof the condenser plate 83 toward one or the. other of the stationary plates 84 or 85 results in the' energization of the corresponding tube 80 or 81,

thereby causing it to operate upon. an amplifier.

unit 88. The amplifier 8-8 is in turn connected with the generator 80 and operates to establish a field current in th generator in direction to generate an output current of required polarity to caus the spindle feeding motor 10 to exert'torque effecting movement of the cutterspindle in the direction and to the extent required in following the contourof the pattern 33.

An electromagnetic brake mechanism 90 is disposed to act upon the spindle moving shaft H and. is interconnected with the tracer. mechanism in such manner that the brake is applied at the proper times for checking excessive movement of the spindle quill in either direction.

Electrical energy for operating the several electric motors, the electromagnetic brake and other electrically operated elements of the tracer control system, is derived from a current source indi-.

cated in Fig. 2 by the line conductors LI and L2.

As shown, the conductors LI and L2 are connected to a main switch 93 from which feed lines 94 and 95 distribute the current to the various pieces of apparatus.

The cutter spindle rotating motor 35, for ex-- ample, may be supplied with energy by a circuit leading from the feed line 94 through a controlling switch 96 and a conductor 91 that leads to the one terminal Of the motor 35, 'the'other terminal'being connected by a return conductor 98 to k the other feed line 95.

The current supply for th tabl driving motor 40 and the associated cross feeding mechanism is obtained through a circuit which leads from the feed line 95 by way of a conductor I M to a control switch I02. From'the control switch I02 a conductor I03 leads to a shunt field winding I04 of the motor 40 which is connected by a return conductor I05 to the feed line 94.

The control switch I02 is also connected by a conductor I06 to a switch contactor blade I01 01' the double-pole, double-throw reversing switch 44. With the switch blade I01 in the position shown in full lines in the drawing, the circuit continues through a conductor I08 to an armature I09 of the motor 40 and thence through a conductor H0 to the one terminal of the solenoid that actuates the cross feeding Jogger mechaeffecting a line feeding movement of the table 25 in a predetermined direction. I

When the table arrives at the end of its line feeding movement, the reversing switch 44 is moved to the dotted line position by operation of the reversing dogs, as previously explained. With the reversing switch blade I01 in the dotted line position, current from the feed line 95 flows through the switch blade to'the conductor III and thence through the solenoid 55 and the conductor -I I 0 to the motor armature I09 from which the conductor I08 then connects with the other switch blade I I2 leading to the return conductor I I3. This results in current flowing through the armature I09 in reverse direction, and since the current through the shunt field I04 continues in the same direction, the direction of rotation of the motor 40 is reversed to effect the desired line feeding movement of the table 25 in the opposite direction.

At each reversal of the motor 40, the reversing switch 44 opens the circuit momentarily thereby de-energizing the crws feed solenoid 55. This permits the spring 58 to raise the armature plunger 54 thereby causing the pawl 53 to move upward and engage another notch of the ratchet wheel 5I. When the switch 44 is then closed in the other position, the solenoid 55 is again energized and operates to move the armature plunger 54 downward thereby rotating the ratchet wheel. 5I to effect a cross feeding movement. In order that the cross feeding movement may be effected in either direction, the ratchet wheel 5| is preferably connected to the cross feeding saddle screw 52 by means of suitable reversing mechanism controlled by a reversing lever" H6. The reversing mechanism is so arranged that when the reversing lever H6 is in the position shown in solid lines, the saddle 24 will be fed transversely in one direction, while with the lever I IS in the position shown in dotted lines, the saddle cross feeding movement will occur in the other direction. The amount of the cross feeding movement may be regulated by adjusting a screw III at the bottom of the solenoid to change the stroke of the plunger.

As previously mentioned, the tracer mechanism which cooperates with the pattern 33 is of the variable capacitance type in which the movable condenser plate 83 operates between the two stationary plates 84 and 85 under the influence of the tracer point 3I. As shown in Fig. 2, the tracer point orstylus 3I is mounted on the lower end of a spring retained actuating rod I I8 within the tracer sleeve which is disposed to respond axially to the movement of the tracer stylus 3|. The movableplate 83 is retained in a pivoted holder II9 rested upon the rod H8 and is insulated therefrom by an insulator I20. The two stationary plates 84 and 85 are adjustably mounted in the tracer box 8| in parallel relationship with the movable plate 83 which is interposed between them in a manner such that a deflection of the tracer stylus will cause the plate 75. 83 to deviate from a neutral position, toward the variation of the spacing between the movable plate It and the two stationary plates '4 and II results in an increase in the capacitance between the two plates which come closer together, according to the formula kA cwhere C is capacity, It is constant equal to l for air,'A is plate area and t is dielectric thickness or plate spacing.

Energizing current for the tracer mechanism circuit is provided by an oscillation generator III to which a conductor I23 extends from the feed line 95. A line I24 extends fromthe oscillation generator to the negative terminal of a bias battery I25 from the positive terminal of which a conductor I26 leads to the movable plate 83. As previously mentioned, the stationary plates 84 and I! are connected to the two electronic tubes I8 and s1, the connections being effected by conductors I21 and I28 respectively through which the tracer circuit is completed.

The two electronic tubes It and C1 in the control circuit serve to detect and amplify electrical conditions and changes thereof set up in the tracer circuit by operation of the tracer mechanism. The tubes shown are of the triode type,

each having a grid, plate or anode, filament. and

cathode. A filament III in each of the tubes may be energized by an independent electrical supply source (not shown) in the usual manner. A cathode III in the tube 81 is connected by a conductor In with a resistance I33. cathode I34 in the tube 88 is connected by a con ductor I35 with a resistance I". These resistances are'both connected to the feed line OI by conductors I81 and I38, respectively. The triode tube 8'! serves to detect the need for and relays an amplified electric tracer current for regulating the generator output corrective current in manner to exert upward movement feed motor torque, according to the charged condition of ca-v pacitance established by the movement of tracer plate "relative to plate 8!. The tube .6 acts in the same manner from the charged condition of capacitance established by tracer plates 43 and 84, to control downward movement feed motor torque.

' The conductor I21 15' connected from the "down plate 84 to a secondary winding MI in a feed back transformer I 42 from which a conductor I4! is connected to a grid I44 in the tube". An anode I45 in this tube is connected by conductor I46 to the power amplifier I. which serves to magnify the down control current 'to the extent that when transmitted to a control field I41 in the generator ll, it efiectively controls the polarity and output of the generator. Likewise the line I28 connects the up" plate in the tracer mechanism with a secondary winding Iii of a feedback transformer III from whicha conductor I53 is connected to a grid I54 in the triode tube 81. An anode IS! in the tube 81 is connected by a line I 58 to the power amplifier 88 where the upward" electric control current is magnified to the extent of eflectively controlling the polarity and output of the generator corrective current as the control current is transmitted to a second control field II'I thereof.

Likewise. a

l0 Asshowninthecircuitdiasramaconductor III extends from a terminal of the power amplifier II to the control field III while another'termina1 of the amplifier is connected to the control field I4'I in the generator by a conductor Ill. The other side of both of the control field windings I41 and III are united by a common return wire III to a return terminal of the power amplifier ll. Power for the amplifier I8 is ob tained from the feed lines 04 and II through the conductors l1 and II to which are connected two conductors III and III, respectively. leading to the amplifier. When the motor switch It is closed to start the motor 3!, the electrical power circuit to the amplifier 80 is also completed and the control circuit is thereby rendered operable.

The filaments I80 in the two tubes, when electrically energized, serve to heat the cathodes IiI and I34. The cathodes then emit electrons which carry currents from the anodes to the cathodes respectively, the anodes being positive with respect to the cathodes and the currents being subject to control action of the grids. The correct grid bias voltage is determined for emcient operation of the circuit and is selected at the bias battery I". Resistances Ill and I" connecting the cathodes with the feed line independently cancel part of the grid bias voltage and stabilize the respective performances of tubes 80 and 81 by degenerative action. The respective currents in the anode circuits change as the voltages impressed on the grids are varied. Variation of anode current corresponds inversely with grid voltage negative potential (with respect to cathode) and variation thereof is effected by changes in the respectivecapacitances and in condenser charges stored between the movable tracer plate 83 and the respective adjustable plates 84 and II, as well as by feedback voltages induced inthe secondary windings "I and lil. The grid I44 in the tube" is connected, through winding I, with the plate 84 in the tracer mechanism and the voltage variations thereof effects the anode current variation to control "downward mechanism. In a like manner, changes in voltage across the movable plate 8! and the plate I! in the tracer mechanism will efiect a grid voltage variation in the tube 81 and a corresponding plate current variation to control upward movement torques of the mechanism.

This grid voltage variation, as caused by tracer action, is accomplished through grid rectification phenomenon as exemplified in what is termed grid leak power detection in the electronic art. The control here afforded is accomplished by variation in capacitance between plates N and I4 and plates '3 and BI under movement of the stylus SI and the charges thereof. These capacitances are shunted respectively by grid leaks III and Il4. During each respective positive half cycle of voltage from the oscillation generator I22, an impulse of grid current fiows to charge the tracer capacitances negatively. Part of these accumulated charges, representing a portion of the respective negative grid biases, escapes through the grid leaks during each succeeding negative half cycle of oscillator I22. The losses however are immediately replenished on the following positive swing.

The desired action is predicated on fixed values of capacitance between the plates 43 and I4 and the plates 83 and 8!. As these capacitances vary according to tracer stylus deflections however,

movement torques of the tracer 11 the voltages across the plates change according to the formula where e is voltage, Q is charge and C is capacitance. The rate of charge dissipation through the grid leaks I63 and I64 also changes according to Ohms law for current flow, and the respective grid potentials of tubes .88 and 81 are thereby adjusted for a change in tracer stylus deflection. The time constant of the circuit composed of grid leak and grid condenser (tracer capacitance) is so predetermined that as the capacitance for one set of condenser plates is decreased by stylus deflection the rate of loss of charge through the grid leak will allow the controlled grid potential to become less negative at a rate most favorable for the control desired.

When the movable tracer plate 83 is in a central position between the two stationary plates 84 and 85, the two capacitance values are equal and the grid voltages of both of the electronic tubes 86 and 81 are likewise equal. The tubes balance out to cancel generator output, and the feed motor 18 exerts no driving torque. For this balanced condition, the anode control current flow consists of positive half cycle peaks at the frequency of the oscillator I22 and is equal in the two control field circuits. The anode currents are preferably of low average value when balanced and depend on grid circuit design and adjustment.

The generator 88, which is driven by the motor 35 as previously mentioned, includes an armature I68, 2. series field I69, and a shunt field I18, in addition to the two control fields I41 and I51 already mentioned. The shunt field I18 is connected across the armature terminals while the series field I89 is connected in series with the armature. The electric control currents flowing in either or both of the control fields serve to control the corrective current output of the generator. A change in the predominance of the magnetomotive force of one field over that of the other field efiects a corresponding change in the magnitude and polarity of the generator corrective current output. This reversible output is adapted to regulate the driving power sources of the machine, namely the spindle feed drive motor 18 which is arranged to raise and lower the tracer sleeve and cutter spindle, and the table feed drive motor 48 which is arranged to effect the lon itudinal movement of the table in either direction, the table movement in turn functioning to regulate the cross feed indexing mechanism 58 attached to the saddle screw to efiect movement of the saddle and consequently the sidewise movement of the table, as previously explained.

As shown in the circuit diagram, one lead wire I14 from the generator is connected to one end of a center tap resistor I15 and the circuit leads thence from the outer end of the resistor through a conductor I16 to one input terminal I11 of a bridge rectifier I18. A second input terminal I19 of the rectifier I18 is supplied with current from the generator 88 through a conductor I88 leading from the series field I89 to a feed motor armature brush I8I, the circuit leading thence from a brush I82 through a conductor I83 to the input terminal I19. The rectifier I18 in the generator output circuit maintains the flow of current unidirectional through field windings I84 and I85 respectively of the spindle feed motor '12 18 and the table feed motor 48 which are connected in series through a conductor I86. Reversal in the direction of rotation of the motor 18 will occur upon each reversal of polarity in the output flow f the generator 88. Thus, if the generator output line I14 is rendered positive by a corrective current flowing from the generator 88, the corrective current can flow only from the terminal I11 oi the rectifier through a branch thereof connecting with a terminal I98. The flow will then occur through a conductor I9I to a resistor I92, thence through a. line I93..,to the control field I 85 in the table 'feed drive-motor 49. the line I86 to the field i84 in'the-spindle feed motorand then by Way of a line I94 to the return terminal of the rectifier. From the return terminal, the corrective current will flow through the lower branch of the rectifier I18 to the terminal I19 and thence through line I83, the spindle feed motor armature brush terminals I82 and I8I and fina1ly.through line I88 to com-' plete the circuit to the generator.

. output current. The corrective current will then fiow through thisline to the spindle feed motor .armature brush terminals I8I and I82, the line I83, the terminal I19 of the bridge rectifier and the upper branch of the rectifier to the terminal I98. From this terminal, the current flows through the conductor I9I, the resistor I92, the line I93 to the control field I of the table feed motor, and thence through the line I86 to the field I84 of'the spindle feed motor 18. From the field I84, the current returns through the line I98 to the bridge rectifier and finally through the line I16, the resistor I15 and the line I14 to complete the circuit to the generator.

From the foregoing explanation of the two circuit paths, it is clear that the bridge rectifier l18 maintains the flow of corrective current unidirectional through the spindle feeding motor field I88 and the table feeding motor control field I85 regardless of reversal in polarity of the generator output. Consequently, reversal of the corrective current flow from the generator to the armature brush terminals I8I and I82 and hence through the armature of the spindle feed motor 18 will result in effecting a reversal in the direction of torque effort of the motor. Reversal of the corrective current effects reversal in polarity of the voltage drop across the resistor I15 but the rectifier unit I18 effectively prevents reversal in the direction of flow of the current through the spindle feeding motor field I 88 and the table motor control field I85.

As previously explained, the controlled current flow through the armature and field of the spindle feeding motor 18 is varied by action of the generator 88 under the control of the tracer mechanism in accordance with the amount of feeding movement required in following the contour of the pattern. Accordingly, the rate of spindle feeding movement may vary continually as the tracer traverses the pattern. In order that the resultant feeding movement between the cutter 88 and the work piece 32 may be maintained substantially constant, the table feeding motor 48 is controlled inversely to the spindle feeding motor. To this end, the same current which flows through the spindle feeding motor and the unidirectional circuit controlled by the rectifier I18 is led through the table motor control field I85 as previously mentioned. To provide the defeed motor, the reducedgenorator output corrective current flowing through the control field ill of the table feed motor will belesseifectiveinbucklngtheourrentinthe "fieldlilandtheresultwillbeanlncreaseof throughout a given tracing operation. The finish on the work piece is uniform throughout since the combined feed rate of the cutter spindle and the work piece tends to be uniform at all times.

When a steep portion is reached in the contour of the pattern the tracer and cutter spindle feed rate is increased while the table feed rate is reduced. Likewise, when a fairly level area in the contour of the pattern is reached, the vertical movement of the tracer and cutter is reduced considerably, and the table feed rate is proportionately increased to maintain a predetermined cutting rate in the direction of the pattern surface.

A supplemental feedback control to the grids of tubes II, II, Fig. 2, is provided by the transformers H2. Ill. The primaries III, I" are in series and the series circuit is connectable, by switch means including a double-pole doublethrow switch Ill and a polarity switch 2, to selectively include in the series circuit either the upper half of the resistor I II, or the lower half thereof, or a resistor I02, whereby to respectively eilect different control results supplementing the normal control result of tracer .2. Thus the series-circuit conductors I" and "I are con- I motor ll.

nected to the terminals of the double-pole, double-throw switch 2" and the action of such switch may be modified by polarity switch 2 When both switches are positioned as shown in Fig. 2 the conductor II! is connected to the center tap of the resistor III by a conductor III, while the conductor I" connects with the upper end terminal of resistor Ill through a conductor 202, the polarity switch 203 and a conductor 2". This switch connection, provides a first method of control in which the voltage drop across the upper half of resistor I'll is caused to be impressed on the primaries I" and I" of the transformers I42 and III, and when the voltage drop across resistor I'll varies in magnitude or direction the flux in the transformers I42 and It! changes to induce voltages in the secondaries Ill and III thereof respectively to modify the respective control action of the tubes It and 81.

The transformer windings are so connected that for the positions of switches 20', 2 Just mentioned a decreasing down direction current through resistor Ill induces in the secondary III a voltage causing grid I" of tube 81 to become more negative, and simultaneously induces in the secondary ill a voltage causing grid I of tube 8| to become more positive. This same change in grid potentials likewise occurs for increasing up direction current in resistor Ill.

According y with the switches at, m in the position shown in Fig. 2 the resulting feedback to the tubes It and I! provides de-generative action for tube ll andregenerative action for tube It when the torque of feed motor II is either didown direction torque or increasing "up" direction torque as qualified by rate of change of the generator current against time. Conversely the feedback provides regenerative action for tube 81 and degenerative action for tube II when the torque of feed motor II is either diminishin up direction torque or increasing down" direction torque as qualified by rate of change of the generator current against time.

may be said to provide an anticipatory influence for both directions of torque eflort and to effect a modulating or proportioning action of damping. The word "anticipation" as here applied refers to the diminution of torque effort of the spindle feed motor Ill through feedback, and for this case is proportional to the time rate of change of the tracer initiated corrective current, for a given tracer stylus deflection. In this sense the feed motor torque is automatically checked to anticipate" stylus deflection and thereby establish the right amount of correction. v

Since tracer controlled grid voltages are a function, of the position of movable plate '3 between the plates 84 and 85 of the tracer, the anticipator" control aforedescribed might also be conceived of as having its origin in the tracer box II. If the condenser plates 84 and 85 are imagined as being fixed to a common shiftable yoke (not shown) capable of motion in line with the motion of plate 83 and this yoke is caused to follow up a movement of plate 83 so as to partially restore the original relative position of plate 83, but only during a change of their relative positions, in accordance with the time rate of change of generator current, then this same described anticipator" feedback action may be achieved.

For certain types of tracer cuts, with special reference to finishing cuts. this mode of operation is especially to be preferred, since overtravel of the cutter must be held to an absolute minimum as the final skin cut is made over the work to obtain a finished surface.

Another or second mode of operation is provided to secure control of a different nature which is to be preferred for roughing cuts because of the particular ability to exert cutting pressure as well as quickly relieve the cutter of the work and at the same time accomplish stable operation. This other mode of operation is effected by throwing the switch 200 to its lower position whereby conductor i9! is joined to a conductor ill thereby making connection with one terminal of the resistor "2, and whereby Thus the described first method of control satisfaction of the tracer conductor I91 is joined to a conductor 206 thereby making connection with the other terminal of resistor I92. In this case, similar to the previous case, when the current through resistor I92 changes, there will be induced in the secondaries MI and II feedback voltages proportional to the rate of change of current, but in this case the current through the resistor I92 is unidirectional by reason of the rectifier I18.

Thus, by the'proper connections of the transformers I42 and I52, there is provided for either output polarity of the generator 80, a feedback current created by the voltage drop across resistor I92 which renders grid I54 of tube 81 more positive and grid I44 of tube 86 more negative during increasing generator output corrective current, and in opposite fashion the feed back renders the grid I54 of tube 81 more negative and the grid I44 of .tube 86 more positive during decreasing generatorputput corrective current. Thus no matter whether the corrective effort of spindle feed motor be for up torque or down torque, an increase in torque effort is accompanied by regenerative feedback to tube 81 and degenerative feedback to tube 86 and a decrease in torque effort is accompanied by degenerative feedback to tube 01 and regenerative feedback to tube 86.

Here too, we may consider the case where plates 84 and 85 are imagined as being fixed on a common shif-table yoke and moved in accordance with the time rate of change of generator corrective current. If the yoke is moved to increase the spacing between plate 85 and plate 83 for any change in position of plate 83, whether it be toward or away from plate 85, the yoke being restored to its'original position when the plate 83 assumes a fixed position, then the feedback control for this second mode of operation is accomplished in mechanical fashion to duplicate the electrical method described.

This second method of feedback control there fore eflfects a maximum initial corrective current value with a succeeding diminution to normal value for up" direction torque effort as the tracer stylus is deflected upwardly and then maintained constant. and effects a slowly rising delayed corrective value with a succeeding rapid rise to normal value for down direction torque effort as the tracer stylus is deflected downwardly and then maintained constant.

Control of this nature provides more than normal initial torque for rapid up travel away from the work when the stylus is deflected to cause plate 83 to move away from plate 85, with a succeeding diminution of corrective current and torque of the spindle motor 10, following the maximum initial value. It also afiords less than normal initial torque whereby to cause a checked initial downward movement of the spindle toward workpiece, as the tracer stylus deflection is lessened to cause tracer plate 83 to move away from plate 84, with a succeeding increase in corrective down current and torque of the spindle feed motor 10, thus providing torque effort to back up the cutter with pressure for roughing cuts. This performance precludes overtravel in a downward direction by the surge of initial up" torque and preventsundercutting the work inside the definite envelope of the motion or outline of the travel as established by the configuration of the pattern.

Another or third mode of feedback operation is provided, in which the series circuit of the transformer primaries I95, I96, Fig. 2, includes the lower position of the resistor I15. Thus with the switch 200- in the upper position as described for the first mode of operation, the polarity switch 203 is thrown to its upper position instead of the lower position shown, whereby toconnect with a line 201 instead of the line 204. The effect of changing the position of polarity switch 203 is to reverse the polarity of the conductors I91 and I99 and of the feedback grid bias on tubes 86, 81, as compared to the first described feedback method.

This third mode of operation, as might be expected, is opposite in effect to the first mode of operation because of the effect of reversed feedback voltages on the instantaneous grid voltages. Briefly, the resulting feedback to the tubes 86 and 81 provides regenerative action for tube 81 and degenerative action for tube 86 when the torque of feed motor 10 is either diminishing down di rection'torque or increasing up direction torque" and provides degenerative action for tube 81 and regenerative action for tube 86 when the torque of feed motor 10 is either diminishing up" direction tor' ue or increasing down direction torque. Similarly to the previously described methods, in each instance the intensity of the instant feedback result varies directly as the rate of change of current in the resistor I15.

For this third method of control the feedback operates to initially increase the normal value of any tracer-controlled change of current from generator 90 to motor 10, whereby to initially increase any corrective torque exerted by the motor.

Since the feedback obcurring in push-pull fashion is in an aiding sense for each movement of ceived'of as moving in accordance with the time rate of change of corrective current together with the condenser plates 84 and 85, in over center style, to set up a swing from one extreme position to the other, and simulate the feedback action just described. The oscillations in tracer current thus set up by the third method of feedback would normally tend to produce a mechanical oscillating or hunting condition in the vertical movements of the tracer sleeve and cutter spindle, except for the stabilizing influence of the previously mentioned mechanical brake 90 and its control apparatus. Since the speed of vertical response and follow is greater under this arrangement, more rapid rates of line feed are permitted. This mode of operation may be preferred especially where a softer material such as brass is being machined.

In order to prevent hunting and overtravel of movement of the cutter and the tracer under conditions in which the feedback circuit does not provide the anticipatory influence occurring under the first mode of operation, the electromagnetic brake 90 is arranged to be engaged automatically during deceleration of the feed motor and to be released whenever the feed motor exerts torque for effecting an accelerating action. As shown in the circuit diagram, the brake 90 is under the control of a direction conscious acceleration-deceleration brake switch 2 I 0 mounted on one end of the tracer frame 21 and operatively connected to the shaft II. The electromagnetic brake 90 is of the magnetically released type in that it is arranged to be disengaged when the magnet is energized and to engage automatically upon de-energization of the magnet. Current for energizing the brake magnet is derived from the feed line from which a conductor 2 leads to 17 the brake switch 2l0 that is in turn connected to the winding of the brake 90 by a conductor 2|2. A return conductor 2| 3 leads from the brake winding to the conductor 91 of the spindle driving motor circuit through which a return connection is effected by means of the switch 96 to the feed line 94, the arrangement being such that the brake circuit is de-energized whenever the switch 96 is opened to stop the spindle driving motor 35.

The brake controlling switch 2 I is so arranged that it moves to closed position upon acceleration of the shaft H, the brake 90 thereby being energized for movement to its disengaged position. Upon a decelerating action of the shaft H, the switch 2 I 0 opens, thereby de-energizing the winding of the electromagnetic brake 90 and permitting the brake to engage to assist the decelerating action and prevent overruning of the motor which might otherwise result in inaccurate positioning of the cutter 30.

The brake switch 2l0 is equipped with means responsive to changes in direction of rotation of the shaft H and operative to condition the switch for responding to acceleration or deceleration of the shaft in accordance with its direction of rotation as is more fully explained hereinafter in connection .with Figs. 7 and 8. To permit slight movement of the shaft H in reverse direction for actuating the direction conscious element of the switch 2), the brake 90 is mounted in manner to provide a small amount of lost motion. To this end, the brake 90 is fitted with a reaction torque arm M5, the extending end of which is positioned between a pair of adjustable abutment screws H6. The abutment screws are spaced to permit limited reverse movement of the shaft it without interference from the brake, of an amount sufficient to condition the direction responsive element of the switch 2| 0 to eifect release of the brake in response to acceleration in the reverse direction. After the lost motion has been taken up, the torque arm 215 again engages an abutment screw 216 in manner to resist further turning movement when the brake is engaged during the next decelerating action.

It is to be understood that the brake reaction arm lost motion arrangement shown in Fig. 2 represents only one method of accomplishing actuation of the direction responsive element of the acceleration brake switch upon reversal of the feed motor and prior to brake release, and that it serves to illustrate the principle only.

Another method of accomplishing actuation of the direction responsive acceleration brake switch 2l0 prior to brake release, comprises mounting the brake switch for actuation directly on the feed motor armature shaft and then transmitting the corrective feed motor torques through a lost motion elastic coupling (not shown) to the shaft H. In this case, the reaction arm 2|5 of the brake 80 is tightly clamped between the abutment screws 216 to eliminate lost motion.

In utilizing a relatively small cutter to machine a work piece from a pattern of the shape shown in Fig. 2, for example, it is not feasible to attempt to remove metal to the full depth of the cut in one stroke. Accordingly, it is desirable to remove the excess metal stock by a series of layer cutting operations of predetermined limited depth, the depth of cut depending upon the type and size of the cutter, the hardness of the material and other factors.

For thus limiting the maximum depth of the cut, the tracer stylus 3! is provided at its upper end with an extending flange 220 that is adapted to cooperate with an adjustable tripper 22l. As shown in the drawing, the tripper 22! is ad- Justably retained in a clamp 222 attached to the frame 21'adjacent to the tracer sleeve 29. The tripper is provided with an arm disposed to contact the lower surface of the flange 220 in such manner that when the cutter and tracer are fed downward to the predetermined depth the flange will engage the tripper and arrest movement of the tracer point, thereby causing the tracer mechanism to operate in a manner to stop the downward feeding movement.

If the table 25 is assumed to be moving to the left in Fig. 2, the tracer point 3| and the cutter 30 will move downward in following the sloping surface of the pattern 33. As there shown, the" cutter 30 has almost reached the maximum depth of the cut indicated by the horizontal line on the work piece, the vertical distance yet to be traveled being indicated by the distance set off by the arrows 223. This distance of further downward movement is the same as the distance indicated between the arrows 224 representing the space betwen the flange 220 on the tracer point and the tripper 22l. When this further distance of downward movement has been traversed, the flange 220 will engage the tripper 22! thereby stopping downward feed movement and the table will continue in its longitudinal feed movement to cause the cutter to machine a horizontal surface on the work piece.

After the entire surface of the work piece 32 has been machined to the predetermined limited depth, the clamp 222 may be loosened and the tripper 22l moved downward a distance equivalent to the depth permissible for the next succeeding cut, whereupon the tracing operation may be repeated to remove another layer of stock from the work piece.

A second electrical circuit exemplifying the principle of reversible polarity tracer control is shown in Fig. 3. In this case a variation in the mechanical tracer structure is shown associated with a work table that may be provided with table and saddle drive structures which are identical with those of the machine shown in Figs. 1 and 2. The attachment shown includes a working head or frame 233 that is slidably supported for vertical operation On two ways 23!] and 23f of a suitable supporting base plate and slidably retained in position by two studs 232 extending through two vertically disposed slots in the supporting frame 233 of the attachment. A rotatable cutter spindle assembly 234 is disposed on the left end of the frame 233 while a tracer assembly 235 is disposed on the right end of the frame. The cutter 235 is locked in a taper-bored spindle 231 by means of a drawbar 238. The spindle 231 is appropriately geared to a variable speed electric motor (not shown) enclosed within the spindle assembly 234. Variations in the speed of the cutter operating on the work piece 233 are effected by controlling the electrical input to the spindle motor. i

The tracer assembly 235 includes means for retaining a tracer stylus 240 in operable relationship with a pattern 2 clamped on a work table 242 together with an electrical mechanism designed to convert the tracer stylus movements into corrective electric currents. A tracer rod 245, slidably extends through a ball 246 with a. shoulder on the rod biased against the upper st le of the ball 246 by a spring 241 reacting between an abutment on the frame 235 and a washer 248 rigidly amxed on the rod 245. The ball 246 is suspended in a ball socket 249 integrally formed in theframe 235 as is shown in Fig. 3. The slidable relationship between rod 245 and ball 245 permits axial movement of the stylus 248 and rod 245 against the bias exerted by spring 241 so that the shoulder on rod 245 always returns to a normally seated position against the top surface of ball 248 when permitted according to the manner in which stylus 248 engages the pat- -tern24l.

. faced, insulated contact 256 is attached to an extension arm 251 fastened to the bell-crank and f is constrained for movement between a pair of ,vibratory contacts or armatures 258 and 259. These two armatures are fulcrumed in the tracer frame 235 with the contact surfaces thereof in alignment with the surface of the movable contact 256. Springs 258, biased between the arms.- tures and adjustable screws 28L are set in the tracer frame 235 to oppose the magnetic attraction of electromagnets 262 and 263 arranged to act upon the armatures respectively. Two pairs of limit abutment screws.284 and 285 mounted to engage the ends of the arms of the vibratory armatures 258 and 259 tend to restrict the swing of the armature contacts during the cyclic vibrations.

The electrical circuit consists of two portions, a control circuit and a generator output circuit. The corrective electrical current originates in the control circuit and is transmitted to control field 218 and 2H of a generator 212. A line 213 extends from a direct current supply source to the movable contact 256 in the tracer assembly.

- This contact is moved by the axial and sidewise movement of the tracer rod 245' as the tracer stylus 248 retained thereon responds to variations in the contour of a given pattren. A predetermined amount of deflection imparted tothe trac ing stylus will effect a balanced condition .be-

tween the movable contact 256 and the two vibratory contacts 258 and 259. This position of the tracer rod 245 and bell-crank extension arm 251 is designated as the neutral position, since no electrical energy or a minimum amount thereof will then pass between the contact 256 and either of the vibratory contacts 258 and 259. However,

.as the pressure on the stylus is decreased to effect a movement away from the neutral position, the. movable contact 258 will tilt to the left, engage the vibrating contact 259 and the resultant generator output corrective current flow will effect a downward movement of the tracer and cutter.

- Likewise, as the tracer stylus is deflected to effect return to the neutral position and beyond to the other side of neutral. contact 258 will tilt to the right, engage the vibrating contact 258 and the resultant generator output corrective current flow will effect an upward movement of the tracing stylus and cutter. In either case the tendency is to react in such manner as will reestablish the neutral condition between the contacts. The tracer control currents thus initiated by action of the tracer stylus in following the contours of a given pattern are transmitted to the control fields 218 and21'l in the generator 20 212 to effectively regulate the polarity and magnitude of generator output corrective current.

The generator 212 is driven by a constant speed electric motor 215 coupled to it. A flywheel 216 is interposed between the generator and motor in order to stabilize the inertial forces and equalize the rotative speed of the generator. Variations in the speed of the generator, if permitted, would tend to distort the effect of the electric control currents flowing in the control fields of the generator and therefore cause a distorted output.

The generator control fields consist of two separate windings 218 and 2' adapted to control generator output polarity with the field winding 2" predominating for down feed motor torque and the field winding 218 predominating for "up" feed motor torque. One end of each of the field windings is connected to a line 211 which extends from the supply source. A line 218 from the control field 218 leads to the left or down vibrating contact 259 and serves to conduct the "down" electric control current to the generator while a line 219 from the other control field 2' to the right or up contact 258 serves to conduct the "up electric control current to the generator. A condenser 288 in series with a resistance 28I, interposed between the line 213 and the line 219 serves to minimize the sparking which ordinarily occurs each time the movable contact 258 engages the contact 258 within the tracer mechanism. For the same reason a condenser 282 in series with a resistance 283 is connected between the line 213 and the line 218.

Vertical positioning of the tracer attachment head or frame 233 is accomplished by means of a motorized screw shaft 285 rotatively supported on a base plate 288 of the attachment. The screw shaft projects vertically through a threaded nut element integrally formed through the center of the adjustable frame 233. The screw shaft 285 constitutes an extension of an armature shaft that is journalled in a series wound electric motor 288. An armature 289 within the motor 288 is mounted directly on the shaft 285. A reverse acting electric brake 298 located on the shaft directly below the motor 288 serves to brake the rotating screw shaft 285 whenever the rotative torque of the shaft decelerates or ceases rotation. A plate 29| is slidably keyed to the shaft for axial movement to form a single rotatable unit. This plate is urged by a spring 292 to effect engagement of the braking surface thereof with a stationary brake surface 293 parallelly disposed to the plate 29L at right angles to the axis of the screw shaft 285.

.A line 295 is connected to one of the generator rotor brushes 298 and to a resistance 291. From the resistance 291, a line 298 extends to a pair of parallelly connected rectifiers 299 and 388. From the rectifiers the energy is directed to the motor. The two rectifiers serve to render the circuit to the feed motor 288, polarity conscious and thereby eflect a reversal of the feed motor torque for each corrective current polarity change. Energy is conveyed from the rectifier 299 via a wire 38! to a series field winding 302 connected in series with the armature 289 of the feed motor 288. The circuit is completed from the armature 289 of the feed motor to the generator armature -brush 383 by a wire 384. With the current flow 21 240 and cutter 236 into contact with the pattern 2 and work piece 239, respectively. If an upward movement of the tracer attachment is demanded by the tracer mechanism, the generator output corrective current will originate at the armature brush 303 and flow through the wire 304, the armature 289 of the motor 288, a series field 305, a line 306, the rectifier 300, the line 298, the resistance 291, the line 295, and the brush 296 to complete the circuit. Separate series fields are provided in the feed motor 283 in order to obtain a torque reversal thereof for each polarity change in the generator output corrective current.

The two contacts 258 and 259 in the tracer mechanism 235 are caused to be vibrated for the purpose of affording a modulated resultant output of the generator 212 in the following manner: A line 307 extends from the generator output line 295 to a coil 308 surrounding the electromagnetic core 262 designed to magnetically vibrate the armature of the "up contact 258. The coil 308 is series connected by a line 309 to a like coil 3l0 surrounding the electromagnetic core 263, positioned to magnetically vibrate the armature of the down contact 259. A line 3 from the coil 350 to the line 298 completes the circuit. The voltage drop across the resistance 291, as set up by corrective current flow through the resistance, serves to create a flow of current through the electromagnetic coils. The cyclic flow of generator output corrective current in either direction through the feed motor circuit causes both coils 308 and 3l0 to intermittently become energized and de-energized, and, consequently, magnetically attract and release the contact arms 258 and 259 to create a vibratory action. These vibrations are synchronized with the frequency of the generator output corrective current reversals.

Provision is made to brake the torque of the driving motor 288 and the vertically disposed screw shaft 285 driven thereby. As previously explained, the brake mechanism 290 is positioned directly below the motor while the brake switch 250 is disposed above the motor. Both are operably connected to the shaft 285, as previously explained, in connection with Fig. 2. A line 3i3 connects the current supply source with the brake switch U0. The switch 2I0 and the brake mechanism 290 are serially connected by a conductor 3. The conductor 3" is connected to an electromagnetic brake coil 315 which, in turn, is serially connected to another brake coil 3l6 by a conductor 3H. Coil 3I5 is wound on an iron core 3&8 while coil 3l6 is wound on an iron core M9. The cores are diametrically disposed with respect to the shaft 285 within the brake mechanism 290 and fixedly retained to impart a magnetic attraction to the rotatable brake plate 29L The circuit from the coil 3|6 is completed to the supply source by a conductor 320. This supply source is also used to energize the spindle drive motor (not shown) built into the spindle assembly 234.

Whenever the switch circuit within the switch M is completed, brake current flows to energize the electromagnetic coils 3l5 and 3l6. Magnetic fields which emanate from the electromagnetic cores M8 and 3I9 respectively upon-the occurrence of a current flow through the coils magnetically draw the plate 29! upwardly away from the stationary brake surface 293 against the bias of the spring 292 to release the braking torque normally existing through engagement between the brake plate 29! and brake surface 293.

When the switch circuit in the brake switch 2 l 0 is opened, brake current ceases to flow and the electromagnetic fields collapse to permit the comparativel stiff spring 292 to instantly urge engagement between brake plate 29! and brake surface 293 to re-establish normal braking torque and resist any rotation of brake plate 29] and the shaft member to which it is keyed.

The application of magnetic drive clutches to a tracer system together with a variation in the tracer vibrator circuit is exemplified in Fig. 4. The frame structure and the tracer mechanism is identical to that shown in Fig. 3 and for purposes of identification bears like numerals. As in Fig. 3. the control circuit is fed by the line 213 extending to the movable contact 256 from the supply source, and the lines 219 and 219 extend from the vibratory contacts 259 and 253, respectively to the generator control fields 210 and 211. The principal difference between the circuits of Figs. 3 and 4 resides in the connections for energizing the electromagnetic coils 303 and 3l0 in the tracer assembly 235. In Fig. 3, the coils 308 and 3l0 are serially connected whereas in Fig. 4 the interconnecting wire 309 is tapped with a line 324 which is connected to the common corrective current feed line 304 that also serves to supply current to clutches 325 and 326. Two lines 321 and 328 connected to the free ends of the electromagnetic coils 308 and 3l0, respectively join them with the corrective current feed lines 306 and 3M to achieve vibrator action in each of the electromagnets 262 and 263 independent of the other. This arrangement affords a connection of the electromagnetic coils 308 and 310 with their respective clutch coils.

The screw shaft 285 rotatably mounted to effect upward and downward movement of the tracer cutter assembly is driven by the magnetically energized, direct-action clutches 325 and 326. A motor 329, connected to an appropriate supply source, is positioned at right angles to the shaft 285 and drives a gear 330. This gear meshes with two identical gears 33! and 332 rotatably mounted on the-screw shaft 285. Each of these gears is keyed to drive an electromagnetically controlled member of the associated clutch unit. These members are adapted to be electrically energized and engage adjacent cooperating clutch plates keyed on the screw shaft in a well known manner. Since the two gears 33! and 332 rotate oppositely from each other they impart corresponding rotative motion to the clutch plates within the clutch units and to the screw shaft. The brake 290 is identical with the one previously described in connection with Fig. 3.

The generator output corrective current is reversible and dependent upon the dictates of the generator field control currents. At the instant when upward motion of the attachment frame is required, the corrective current will originate in the generator and flow along the line 298 through the rectifier 299, a line 30l, to the "up clutch 325 and thence return to the generator via the line 304 to complete the circuit. Thus the magnetic attraction created within the direct action clutch 325 by this corrective current tends to magnetically urge the clutch member and plate together and the member is then able to transmit rotative torque to the clutch plate and to the screw shaft 285. The tracer assembly threadably attached to the shaft will then be moved upwardly since this accelerating torque accomplishes rotation of the shaft.

Due to the initial lost motion interval in which the brake reaction arm of brake 299 functions as the shaft 285 starts rotation for up travel, the direction acceleration brake switch 2l9 will have closed its switch circuit to energize brake 290 for release of brake torque during this upward acceleration movement.

If the next corrective current calls for downward movement of the cutter spindle and tracer sleeve, the current will flow along the line 364 to the down direct-action clutch 326 and then through line 396 and the rectifier 366 back through line 298 to the generator. No corrective current flows in the direct-action clutch 325 at this instant because current flow in this direction through rectifier 299 to line 298 for return to the generator is not possible.

The magnetic attraction thus created within the direct action clutch 326 by corrective current tends to magnetically urge the clutch member and plate together to transmit rotative torque to the clutch plate and screw shaft 285. This torque is decelerating (plugging) in efiect and acts to arrest the existing upward velocity. The first effect of deceleration is to actuate the inertia mass switch of the brake switch 2m and open the switch-brake circuit. Brake 296 immediately responds and assists the deceleration with braking torque until such time as the motion is arrested. The persisting torque of the direct action clutch 326 according to tracer stylus dictates, however starts acceleration of motion in the opposite or downward direction to actuate the friction direction switch of the direction acceleration brake switch 2l6 thereby releasing the braking torque of brake 299 as the switch circuit is closed.

The cycle of motion change from an upward to a downward velocity has thus been followed through. A change of motion in the reverse order according to the tracer stylus demand follows the same sequence of events with the same attendant braking actions during deceleration periods.

The tracer vibrator circuit of Fig. 4 is different from that shown in Fig. 3 in that the voltages for this circuit are taken directly from the generator output rather than from a reversible voltage drop in phase with the generator current. Thus immediately after an increased deflection of the tracer stylus 240 produces an up control current and a corresponding generator corrective voltage polarity is estab lished to energize the up clutch 325, the magnetic field created by the energization of the coil 398 will attract the up contact armature and part the contacts. If the increased stylus deflection persists, the up vibrator circuit serves to instigate a series of up" control currents, each separate and distinct from the other and anticipatory in nature to preclude over correction of the stylus deflection, until the tracer stylus is returned to a predetermined neutral position, rather than permit a sustained corrective current which would subside only after correction is attained with consequent overcorrection.

Likewise if the tracing stylus 249 is released to allow a return to a neutral position and beyond, a down electric control current will result and a corresponding generator corrective voltage polarity output will be created. This generator voltage, serves primarily to cause corrective current flow in the down clutch 326 and through torque effort effect a downward movement of the attachment frame 233. The generator voltage also serves to energize the anticipatory circuit to move the down" contact 259 away from the movable tracer contact 256. A continued deflection of the tracer stylus in this direction will instigate a series of distinct down control currents each resulting in a separate downward torque efiort 0n the screw 285 to secure downward movement of the attachment frame until the predetermined neutral position of the tracer stylus is re-established.

Another variation of a reversible polarity tracer system is shown in Fig. 5. This variation may well be referred to as the opposed generator field type. In this case also the tracer stylus and cutter are both mounted on a single movable frame. They therefore move unitarily in an upward or downward direction i the manner explained in connection with Fig. 3. The means of mounting the frame for vertical movement is not shown since any of a number of well known means may be used.

A tracer stem 34!! is swivelably mounted in a frame 34! by means of a flange 342 and a flange seat 343. A tracer stylus 344 is retained on the lower end of the stem 340 for operable association with a pattern that may be clamped on a work table in a manner previously described. A vertically disposed bell-crank 365, having a. recess centrally located on the bottom face thereof to match a recess on the upper end of the tracer stem 340 which serve as retainer sockets for a ball 366, is adapted to transmit any movements of the tracer stylus 344 to control the position of contacts i the electrical control circuit. A pin 34? on the upper end of bell-crank arm 345 serves as a movable abutment to contact a leaf spring 338 carried by a pivotally mounted armature lever 349 which is normally biased for movement toward the vertical arm of the bell-crank 345 under the action of a coil sprin 350. In addition to serving as a bias to urge the armature spring 348 into pressure contact with the pin 341, the spring 35!] serves to retain the operating members associated with the sty'sis 344 in pressure contact with each other.

The electrical circuit is established between two contacts, namely an insulated stationary contact 352 attached to the frame 341 and an insulated movable contact 353 aflixed to a flexible leaf spring 354, which is fastened to the stationary frame 34l. The spring 354 normally tends to retain contacts 352 and 353 together, however, this action is controlled by the pressure contact of an abutment 355 mounted adjacent to the free end of the armature lever 349. A motor-generator set 351 is used to power amplify the control currents created by the action of the tracer contacts 352 and 353 in terms of corresponding output corrective currents. A motor 358 is connected" to an appropriate supply source 359. Intermediate the motor 358 and a generator 369, a flywheel 361 is keyed to the interconnected shafts of the two units.

This flywheel tends to eliminate variations in speed normally encountered in the operation of a motor-generator set.

Two field windings are contained within the generator, namely, a shunt field winding 862 and a control field winding 363. The control field winding 363 is shunted by a rectifier 364 through connecting lines 365 and 366 to provide a path for control field windin relaxation currents and protect the tracer contacts 352 and 28 363 from excessive sparking. A common negative feed line 361 is connected from one end of each of the two field windings 362 and 363 to an appropriate direct current supply source. The positive side of the supply source is connected to the shunt field winding 362 and the movable contact 353 of the tracer mechanism through a line 368, a tracer electromagnet coil 368, and a line 310. Line 366 and a line 311 conduct the control current from the stationary tracer contact 352 to the generator control field winding 363.

Full control current thus passes through the electromagnet coil 369 to efi'ect vibratory movement for contact 353. The "atonic" type vibrator provides means for eilectively regulating the control current as the frequency of vibration is varied by tracer stylus change of armature biasing through action of the pin 341 on the spring 348.

A condenser 313 is connected to the line 31! and to the line 310 through a resistor 314 and is thus parallelly connected with the tracer contacts 352 and 353. This condenser and resistor serve to minimize sparking and arcing across the tracer contacts. v

An upward slope in the contour of a given pattern encountered while tracing will increase the deflection of the tracer stylus 344 following a given path over the pattern which, in turn, will cause a reduction in the frequency of vibration of the movable contact 353 as the armature lever 34'!) is attracted and released by the control current passing through the electromagnet coil 369 with a consequent diminution in the magnitude of control current. However, if the pressure on the tracer stylus 344 is released, as would be the case if a downward slope of the pattern was encountered while tracing, the frequency of vibration of the movable contact will increase with a consequent increase in the magnitude of control current. This modulated control current governs the polarity and strength of the generator field to regulate the generator output corrective current as to polarity and magnitude.

The shunt field winding 362 is energized from the supply source to establish a constant magnetomotive force for the generator field, to control the output corrective current and consequent feed motor torque for upward travel of the tracing attachment. The control field winding 363 is energized from the same supply source but through the tracer contacts 352 and 353. Thus, if the deflection of the tracer stylus 344 is increased during a tracing operation, the frequency of vibration of the contacts will be reduced and the bucking magnetomotive force of the control field winding 363 counteracting that of the shunt field winding 362 will be reduced so that the shunt field magnetomotive force will predominate to establish a net field strength and govern the output current polarity to effect feed motor torque for upward motion.

Likewise, if the stylus 344 is released at a given point due to a change in the pattern slope, the frequency of the contact vibration will be increased and the bucking magnetomotive force of the control field winding 363, counteracting that of the shunt field winding 362, will be suificiently strong to predominate over the magnetomotive force of the shunt field winding and establish a net field strength for governing the magnitude of the opposite polarity output corrective current of the generator to eflect teed-motor torque for downward motion. When the deflection of the tracer stylus is such that the eflect of the control field winding 363 is exactly equal and opposite to the effect of the shunt field winding 362, the respective magnetomotive forces 01' the two field windings will be balanced and no upward or downward motion will result. The governing action of the generator net field is proportional to the amount of tracer stylus deflection and the result of such net field control will always be to increase or reduce this deflection to a predetermined balanced position.

A series feed-motor 316 is disposed to eiiect the vertical positioning of the tracer mechanism and cutter spindle retained within the slidable frame 34L A line 316 connected to one terminal brush of the generator armature 311 and to a bridge rectifier 318, conducts the output corrective current from the generator to the motor 315 for effecting torque to give upward movement to the tracer mechanism. The generator output corrective current passes through the upper branch of the rectifier 318 to a line 319 connected to a field winding 388 within the motor 315. A line 38l from the field winding 388 reconnects with the rectifier 318 wherein the current flows through the lower branch and a line 382 to the motor armature brush 383 and the armature 384. To complete the circuit, a line 385 connects with an armature brush 386 or the motor 315 and with the other terminal brush oi the generator armature 311. Current fiow through the feed motor 315 in this direction will set up a torque tending to rotate a pinion 381 keyed directly to shaft 388 of the armature 384 in direction to elevate the tracer attachment 341 under the action of a rack 389.

If the generator output corrective current polarity is such as to efiect teed-motor torque for downward feed motion, the corrective current will originate in the generator armature 311 and fiow through the line 385, the motor armature brush 386, the motor armature 384, the armature brush 383 and the line 382 to the bridge rectifier 318. From this point the corrective current fiows through an upper branch of the rectifier, the line 319, the motor field winding 380, the line 381 to the rectifier, a lower branch of the rectifier and the line 316 to the generator armature 311. With this generator output current polarity, the direction of torque exerted by the motor armature 384 is such as to rotate theshaft 388 and the pinion 381 in direction to impart a downward motion to the tracer mechanism and cutter spindle mounted on the frame 34!. In each case the bridge rectifier 318 serves to maintain a unidirectional corrective current flow through the motor field winding 388 while the corrective current fiow through the armature is reversed.

A motor 391 enclosed in the tracer frame 34l is directly connected to drive a cutter spindle 392. An electrical supply source variably controlled by appropriate switches (not shown) serves to efiectively vary the speed of the motor and consequently the speed of the cutter spindle 392 and a cutter 393.

A direct acting brake 394 suitably mounted to act on the motor armature shaft 388 serves to apply a modulated braking torque to the shaft in proportion to the corrective current which accomplishes decelerating feed motor torque. A line 395 extends from the direct acting brake 394 to a terminal on the direction-acceleration brake switch 2 l n and then from another terminal thereof, a line 396 leads to the line 382 while a line Two other terminals or the brake switchIlI are Joined by aiumper conductor 3.

The mechanical and electrical, action of' the direct action brake 334 is different from that previously described in connection with the brake shown in Figs. 2, 3 and 4. The present type of brake 334 is normally in a non-braking position and is designed to exert braking torque during a decelerating feed-motor torque. The brake is connected to the generator output circuit in such manner that the brake is electrically energised to effect braking action whenever the brake switch 2|0, mounted on the-drive shaft an opcrates to close the electrical contacts therein and complete the brake circuit. Switch 2|0 with the connections shown operates to complete the brake circuit during all decelerating periods regardless of the direction oi. feed drive.

Since the brake electrical supply source is taken from across the feed motor armature brushes 3" and 336 the brake current is of varying magnitude in proportion to the feed motor current .and torque. This brake current aflords a brak- 8 torque of magnitude corresponding with feed motor decelerating torque as established by the modulating type tracer through generator field control. The magnitude of the braking torque and feed-motor decelerating torque varies directly as the tracer stylus deflection. The mechanical opening 01' the brake switch 2" dis connects the brake from its electrical supply source to operatively release it for all feed-mote accelerating torques.

The previously described reversible polarity tracer systems involved the use of a motor genorator set or electronic power amplifiers for the purpose of converting the tracer control currents into corresponding corrective electric currents capable of controlling the action of the feed drive motor. However, another specific form of power amplifier known as a saturable core transformer may be used in a like manner with similar results. An electrical circuit incorporating these transformers for the purpose of tracer control is shown in Fig. 6.

Both the tracer and the cutter mechanism are on a single frame 40| which may be appropriately retained for vertical movement on way surfaces in the manner shown in Figs. 3 and 4. The tracer mechanism is enclosed in a protective casing 402. A tracer stem 403 is movably retained in the casing 402 by means or a flange 404 on the stem and a flange seat 405. The tracer stylus 406 fastened to the bottom end of the stem serves to operably engage a pattern to be copied, as shown in Figs. 1, 2 and 3. Any movement of the tracer stylus and stem serves to actuate the tracer mechanism. A spring 403 serves to counteract the axial pressure imparted to the tracer stem by the engagement of the tracer stylus 406 with the pattern and maintains a retentive pressure on a bell-crank 409, a ball and the stem 403. A slight axial movement of the tracer stem is transmitted to the pivoted bell-crank 403 through the ball retained between thznipper end of the tracer stem and the hellor A vacuum tube 4 is fixed within the tracer casing. The tracer mechanism serves to actuate a movable anode 2 within the tube. The vertically extending arm of the bell-crank 403 is designed to abut a pivoted actuating rod 3 supporting the anode M2 and protruding from the bottom of the tube through a flexible diaphragm 7 closed therein are mounted in spaced relationship to the anode 2 which is of U-shapeor channel shape and is slightly larger than the cathode.- Therefore, a movement of the actuating rod 3 will cause the anode 4|2 to move sidewise and partially enclose the cathode or be withdrawn therefrom and, consequently, will cause a change in the effective areas of the anode M2 and cathode 4 I 6 as well as change the spacing between the two.

An alternating current supply source is used to furnish electrical energy. A pair of feed lines 8 and 4| 3 are connected to a primary winding of a step-down transformer 620. A secondary winding in the transformer set is connected to the electronic tube heater M7 by two lines 422 and 423. Heating the cathode dit is desired amount will result in a constant emission of electrons from the surface of the cathode.I The number of electrons which actually reach the movable anode & i 2 will depend on the areas presented by and the spacing between the anode and the v cathode at a given instant.

A suitable bridge rectifier 625 connected to the alternating current supply source furnishes a diline 426 from the feed line M8 through the lower branch 428 of the rectifier 425 and a line 423 to the movable anode 4| 2 in the vacuum tube 4 during a given half cycle. During the subsequent halfcycle, the current flow is from the feed line 489 and the line 421 through a lower branch 430 of the rectifier 425 to the line 429 connected to the anode 4 2. Within the tube, the actuation of the anode by the action of the tracer stylus 406, to which it is mechanically linked, serves to eiiect a corresponding variation in tracer current flowing from the anode M2 to the oathode M6 The path for tracer current flow from the cathode back to the negative terminal of the bridge rectifier 425 is completed through a line 432, a resistance 433, and a line 434.

Two saturable core transformers 435 and 436 are used in this tracer system in place of the motor-generator set used in the previously explained circuits. The substitution of transformers in place of the generating set is advantageous in that all moving parts are eliminated. The two transformers are identical in design, one being adapted to aiford control for the up corrective currents while the other is adapted to afford control for the "down corrective currents neces sary to govern the torque efiort of a feed drive motor 440 mechanically connected to eflect the vertical positioning of the tracer frame 40L The transformer 435 will therefore be referred to as the up transformer and transformer 435 will be known as the "down" transformer.

Referring more specifically to the up transformer 435 the core thereof is made up of a plurality of laminations including four spaced ,legs 4, 442, 443 and 444. The various coil windings are disposed on the four legs. A master winding 445 is mounted on the intermediate leg 442 and connected to the feed lines 8 and 4| 3 by two lines 443 and 441 respectively. A control winding 443 wound on the leg 444 serves to receive the control current that results from the tracer current flowing in the tracer circuit. An output winding 449 is disposed on the leg 44! of the transformer.

Likewise the laminated core of the down transformer 436 consists of a plurality of lamina.- tions including four spaced legs 45!, 452, 453 and 454. A master winding 455 is wound on the transformer leg 452 and electrically connected to receive power from the feed lines 6 and 9 via the two lines 446 and 441. A control winding 456 is wound on the leg 454 and an output winding 451 is disposed on the leg 45! of the transformer 436.

Two triode tubes 468 and 46! are used in the control circuit. The tube 468 is adapted to function in connection with the up transformer 435 and includes an anode 462, a control grid 463, a cathode 464 and a heater filament 465. The second tube 46! functions in connection with the "down transformer 436 and includes an anode 461, a control grid 468, a cathode 469 and a heater filament 418. Both heater filaments 465 and 418 are electrically connected in parallel to the supply lines 422 and 423 through two leads 41! and 412. The two cathodes 464 and 469 are connected together by a line 413 which in turn is joined to a cathode bias resistance 414. A line 415 serves as a return circuit and connects the bias resistance 414 to the negative side of the bridge rectifier 425.

The anode 462 in the tube 488 is connected to one end of the control winding 448 of the 1113" transformer by a line 416 while the anode 461 in the tube 46! is connected to one end of the control winding 456 of the down transformer 436 by a line 411. The other end of each of the control windings 448 and 456 are attached to a line 418 which in turn is connected to the positive side of the bridge rectifier 425. A resistance 488 is disposed to function between the line 415 and the line 418 as a voltage divider with a line 48! adjustably connected intermediate its ends to supply a fixed voltage for grid 466 in the tube 46!.

Movement of the attachment 48! is accomplished through the cooperating relationship of a rack 482 secured on the attachment frame 48! and a pinion 483 keyed to the shaft of the polarity-conscious feed motor 448. The control circuit for this motor originates in the output windings 449 and 451 in the two transformers 435 and 838 respectively.

The ends of the output winding 449 on the saturable leg transformer 435 are respectively connected to the rectifiers 485 and 486 and direct the current flow from the output winding, as indicated by the arrows within each of the rectifiers to a line 481 supplying a feed-motor field winding 488. The other end of the field winding 488 is connected by means of a line 469 to an armature brush terminal in the feed motor 448. The other brush terminal therein is connected by a line 498 to a central tap on the output winding 449 to complete the circuit. The corrective current from the down transformer 436 fiOWs from a central tap of the output wind ing 451 into line 498, thence through the feed motor brush terminals and the armature, through line 489 and a line 49! to one end of a field winding 492 in the motor 448. The other end of the winding 492 is connected by a line 493 to a pair of rectifiers 494 and 495 which in turn are connected respectively to opposite ends of the output winding 451.

A direct acting brake 496 is disposed to function on the shaft of the drive motor 448 in the same manner as was -Dreviously described in detail in connection with the brake 394 in Fig. 5, excep that the electrical supply source for brake control is independent of the tracer control and is constant. Thus a separate power source 491 is provided for the brake 496 and the brake switch 2"] with the electrical connections identical to those shown in Fig. 4. A cutter spindle and cutter 498 are effectively operated at a given cutter speed by a motor (not shown) built into the frame 48! and appropriately controlled by means of electrical switches.

Reverting to the functions of the reversible polarity tracer system as shown in Fig. 6, it is evident that any variations in the contour of the pattern will cause corresponding deflections of the tracer stylus 486 when operating over a given path and engaging the pattern. The tracer stylus, in turn, mechanically actuates the movable anode 4l2 in the tube 4!! and provides the only means of setting up electrical oscillations in the control system. The tracer current flow from the anode M2 to the cathode 4!6 of the tube is varied accordingly and the resulting voltage variations across resistance 433 is transmitted to the grid 463 of the tube 468. While the voltage of the grid 463, relative to the negative terminal of the direct current source, in the present instance the bridge rectifier 425, is varied according to the dictates of the tracer stylus, the voltage of the grid 468 relative to the direct current source is held constant as determined by the voltage divider resistance 488.

With the stylus 486 in a neutral position to establish a balanced condition for zero feed motor torque, the tracer current flowing through resistance 433 determines a certain positive voltage for grid 463 of tube 468 relative to the supply source negative line 434. This positive voltage will be approximately equal to or greater than a positive cathode bias voltage drop across resistance 414 as established by the sum of the two control currents flowing through the common line 413 from cathodes 464 and 469, through resistance 414, to negative line 434 oi the supply source. For this balanced condition, the grid 468 of tube 46! will also have a positive voltage adjustably fixed at the voltage divider resistance 488 relative to the supply source negative line 434, which exactly equals that of grid 463 of tube 468.

The positive voltage of the grids 463 and 468 in excess of the positive voltage drop across the cathode bias resistance 414 represents the net grid voltage, relative to cathode, for each of the tubes and determines the magnitude of the equal control currents flowing from each cathode for the balanced condition described. Also, the magnitude of equal control currents for this balanced condition may be changed by manually adjusting the voltage of grid 466 to a, new setting at the voltage divider resistance 488 since the grid 463 will automatically have its voltage adjusted to equal that of grid 468 by readjustment of stylus deflection.

As the voltage of grid 463 is made more positive by a, deflection of tracer stylus 486 to increase the tracer current through resistance 433, the control current flowing from cathode 464 of tube 468 increases to eifect an increase in the total net control current flowing through resistance 414. The resulting increase in positive cathode bias voltage measured across resistance 414 

